PHILADELPHIA (AP) _ A national weight-loss chain has pulled a drug combination known as fen-phen from its program amid concern that the popular diet drug may cause heart valve damage and high blood pressure.

Nutri/System Weight Loss Centers this week began replacing fen-phen _ a combination of fenfluramine and phentermine _ with a mix of phentermine and the anti-depressant Prozac. The chain said the new drug, called phen-pro, is a safer, cheaper and equally effective alternative for the 9 million Americans who take fen-phen.

Nutri/System plans to switch thousands of patients to the new combination of pills within the next 30 days, company officials said.

The move comes two months after the Mayo Clinic reported 24 cases of fen-phen users developing heart valve damage. A week ago, the U.S. Food and Drug Administration said it had discovered another 58 cases and ordered new drug labels to warn patients.

Although the FDA stopped short of recommending that patients not use the pills, it urged doctors to be cautious.

``We know the risk is very small, but we felt that it was in the best interests of the customers to offer them the best available program,'' said Joseph J. DiBartolomeo, vice president for scientific affairs with the Philadelphia-based Nutri/System.

Both of the appetite suppressants in fen-phen are approved by the FDA for short-term use in dieting, although not specifically for use together.

Like fen-phen, phen-pro has its skeptics.

``The problem is we just don't know if they're safe or not safe. The question is, how much is the public willing to put themselves at risk based on what we don't know?'' said Dr. Lewis J. Rubin, head of pulmonary medicine at the University of Maryland Medical School. ``Let's not embrace the next thing that comes down the pike until it's proven safe and effective.''

Nutri/System prescribed fen-phen to thousands of its 250,000 customers last year. About 8 million people spend nearly $2 billion at commercial dieting centers, including Jenny Craig, Diet Center, Physicians Weight Loss Centers and Weight Watchers International.

Studies in the last year have linked fenfluramine with primary pulmonary hypertension and a rare lung disease.

``I think probably they're making a business decision, not necessarily a science decision,'' said James Merker, executive director of the Association of Bariatric Physicians.